1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a radiation system, and more specifically to a removal technique of dark current noise in a captured image.
2. Description of the Related Art
An imaging apparatus using a large-scale sensor, which is configured by two-dimensionally arranging solid-state imaging elements each made up of single-crystal silicon or amorphous silicon, has been prevalently put into practical use. Such imaging apparatus is used not only to capture a visible light image in, for example, a digital camera but also to capture a radiation image in a medical apparatus.
As is known, in the imaging apparatus, dark current noise is generated due to variations of dark currents of elements even in a non-exposure state. For this reason, when output signals of imaging elements are used intact, dark current noise is unwantedly superposed on effective signal components, thus causing deterioration of image quality.
In order to remove such dark current noise, a method of obtaining a dark image by performing imaging in a non-exposure state (to be referred to as “dark imaging” hereinafter), and subtracting the dark image from an exposure image is known. The invention described in Japanese Patent Laid-Open No. 2003-244557 (to be referred to as “literature 1” hereinafter) has proposed a method of removing dark current noise by performing dark imaging under the same condition every time an image of an object is captured. Also, the invention described in Japanese Patent Laid-Open No. 2008-236661 (to be referred to as “literature 2” hereinafter) has proposed a method of removing dark current noise without performing dark imaging for each imaging by obtaining dark image data in advance. Furthermore, Japanese Patent Laid-Open No. 2009-279042 (to be referred to as “literature 3” hereinafter) has a method of compositing dark image data, which is held in advance before imaging and that obtained for each imaging at a predetermined ratio.
Dark current correction by changing a ratio of image components having specific image information in association with dark image data and other image components is often not successful. For example, in the technique of literature 1, an exposure image and a dark image are both required to be used. However, since each image has further random noise components, these noise components are superposed when the subtraction is performed. As a result, random noise components are increased by a factor of √2 times, thus in fact lowering the signal-to-noise ratio after correction. This influences image quality in a region in which the signal is small and is relatively close to the noise level. Especially, in a radiation imaging apparatus which requires low-dose imaging, such increased random noise influences diagnosis performance.
For example, in the technique of literature 2, dark current noise components can only be extracted before imaging. For this reason, when dark current noise components vary due to a change in operation temperature of the imaging apparatus during imaging or generation of an afterimage caused by exposure, correction cannot be performed with sufficiently high precision. Thus, components which cannot be corrected appear as an artifact on an image.
Furthermore, the technique of literature 3 does not specify that coefficients are set for respective image components of dark image data.